Rapid prototyping using stereo lithography and plastic materials has enjoyed great commercial success. The natural extension of this process to metallic and ceramic materials has been tried using free form deposition techniques. Solid free form fabrication of metallic materials is being developed at several laboratories. One of these techniques is an extension of selective laser sintering (SLS) known as laser engineered net shaping (LENS). Instead of bonding material in a bed of powder, the powder is delivered in a gas jet through nozzles into the focus of a Nd:YAG laser to form a molten pool. The part is then driven on an X/Y stage to generate a three-dimensional part by layer wise, additive processing. Solidification of the molten pool forms a fully dense, free standing deposit.
Free form fabrication of ceramic materials is considerably more difficult. In general, the ceramics considered here are ionic bonded combinations of a metal and oxygen (Al2O3, ZrO2, Y2O3). Ceramic powders cannot be processed in a LENS type system for several reasons. The thermal conductivity is too low to provide an adequate heat sink for the laser beam. The gradients generated in the workpiece are simply too high to allow solidification without cracking these thermal shock sensitive materials. Oxide ceramics must maintain stoichiometry for proper crystal growth. Ceramics lose oxygen when melted, so that a molten pool of appreciable size and duration will certainly result in an oxygen deficient structure. Alumina and zirconia are polymorphic and, on cooling, can be cracked by solid-state phase transformations.
Fabrication of ceramic films using 3D printing/robocasting slurries has also been attempted. However, these methods produce green-state objects that must be fired or heat treated before use that results in dimensional changes for the final product. Other experiments have been conducted using direct fusion of ceramic powder by laser energy in processes analogous to the LENS process. These processes have shown limited success because it is difficult to control the heat transfer from the molten zone, thermal stresses, the phases formed by solidification, and subsequent solid state transformations in the object.
For the past decade, the inventor has been engaged in the study of nucleation and solidification kinetics. Specifically, the functional relationships between supercooling and solidification velocity in a variety of metallic systems, and the role of solute and fluid flow on nucleation kinetics have been examined. The role of supercooling on phase selection and solidification kinetics has been well studied in metallic systems. However, the understanding of these phenomena in high temperature oxide systems lags behind metallics. This is not surprising given the present methods of fabrication of the two classes of materials. Most metallic alloys are formed from the melt by casting or atomization. With the exception of glasses, few ceramics are processed from the melt.
In the study of metallic materials, manipulation of the nucleating phase through supercooling is well established. Studies of the TiAl and NbSi systems revealed that phases formed by peritectic reaction can be nucleated directly from the supercooled melt. The same should be true in the high temperature oxide superconductors (HTSC). The desired superconducting phase YBa2Cu3O7-□(Y-123) forms by two peritectic reactions: liquid+Y2O3□Y2BaCuOx(Y-211) and liquid+Y-211□Y-123. The temperature difference between the high temperature liquids and the Y-123 peritectic phase is 600K. Oxide melts are generally glass formers, so this degree of undercooling can be achieved. In experiments using the aero-acoustic levitator on 2–3 mm diameter samples of Y-123, the tetragonal Y-123 phase was formed directly from the melt. This discovery resulted in a continuing study of the high temperature phase relations and alternate processing strategies for oxide superconductors.
What is needed, then, is a system and method for depositing ceramic and other solid films on a substrate that creates a film that is structurally and dimensionally stable, that does not require post-deposition heat-treating of the film, and that is not subject to extreme process control requirements.
This application outlines an approach to develop thick film deposition of ceramic materials. The key concept is the deposition of supercooled ceramic particles on a substrate. The heating source will be optical and not directed at the substrate. Supercooled particles contain less sensible heat and therefore reduce the heat load on the substrate, minimizing thermal shock. The viscosity of liquid ceramics increases exponentially with supercooling. The solidification velocity increases with supercooling as well. These two factors improve the probability that droplet impacts will spread evenly and solidify quickly. The degree of supercooling has some influence on phase selection. The hypothesis is that fully dense, thick films of ceramic materials can be made by careful control of droplet temperature, size, and velocity.